5o8 



Journal of Agricultural Research 



Vol. in. No. 6 



The relation of the time elapsed since the cessation of rain to the 

 prevalence of ascospores in the air among diseased trees is shown in 

 Table VII. 



Table VII. — Relation of the time elapsed since the cessation of rain to the number of 

 spores failing on an area of I square foot per minute in igi^ at West Chester, Pa. 



PLATES EXPOSED ON SEPTEMBER 18. I9I3 



PLATES EXPOSED ON SEPTEMBER 21. I9I3 



An examination of these tables shows that on September i8 the spore 

 content of the air decreased more or less gradually during the third and 

 fourth hours after the rain, while on September 21 the spore content 

 decreased very abruptly and no spores were obtained after the first hour 

 following the cessation of the rain. The duration and the abundance of 

 the ascospore expulsion on these dates (Table XI) are seen to have 

 differed likewise, and a comparison of the weather conditions gives the 

 probable explanation, since it was calm and foggy on the i8th and hot 

 and sunny with a brisk wind just following the rain of the 21st. Condi- 

 tions following the rain on the i8th were such as to prevent rapid drying 

 of the bark, so that spore expulsion continued during a much longer time 

 than on the 21st, when the bark dried rapidly. Furthermore, the brisk 

 wind of September 21 would tend to disperse the spores very rapidly, 

 whereas the comparative calm of September 18 would be favorable to a 

 more prolonged prevalence in the air near their source. In this regard 



